Stage: 4 Challenge Level: 
Look at the diagram below:
Which part of the diagram represents $85^2$?
Which part represents $65^2$?
Which part represents $85^2-65^2$?
Making and proving conjectures. smartphone. Number bases. Manipulating algebraic expressions/formulae. Generalising. Calculus generally. Difference of two squares. Mathematical reasoning & proof. Place value. Factorisation (algebraic).
Published July 1999,November 2009,December 2009.
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